The conventional thermal printers using a thermal head include a sublimation printer, fusion printer, thermal printer, etc. The thermal head used in these printers includes a plurality of heating elements arrayed linearly, energization of each of these heating elements is controlled correspondingly to a gradation level and heat energy thus developed is used to make print on printing media of different types.
The thermal printer will be explained herebelow. In the thermal printer, a printing medium 104 travels being guided by a guide roller 101 and is held tight between a capstan 102 and pinch roller 103, as shown in FIG. 1. Also an ink ribbon cartridge is provided in the thermal printer. It includes a take-up reel 106 and supply reel 107. As the take-up reel 106 is rotated, an ink ribbon 105 wound on the supply reel 107 is taken up by the take-up roll 106. In a printing position where ink in the ink ribbon 105 is to be transferred to the printing medium 104, a thermal head 108 and platen roller 109 are disposed opposite to each other. The ink in the ink ribbon 105 is sublimed by the thermal head 108 and transferred to the printing medium 104.
FIG. 2 gives a detailed illustration of the thermal head 108. As shown, the thermal head 108 includes a ceramic substrate 111, heating elements 113 (will be referred to as “heating element” hereunder) each formed from a heating resistor or the like and disposed linearly on the ceramic substrate 111 with a grace layer 112 laid between them, and a protective layer 114 provided on the heating element 113 to protect the latter. The ceramic substrate 111 is excellent in heat dissipation, and thus functions to prevent the heating element 113 from storing the heat. The grace layer 112 is provided to project the heating element 113 toward the printing medium 104 and ink ribbon 105 in order to putting the heating element 113 into contact with the printing medium 104 and ink ribbon 105. Also the grace layer 112 is a buffer layer to prevent the ceramic substrate 111 from excessively absorbing the heat from the heating element 113. The heating element 113 of the thermal head 108 heats and sublimes the ink in the ink ribbon 105 on the printing medium 104 for transfer to the printing medium 104.
Since the thermal head 108 has a heat capacity and so the heat generated by the heating element 113 is transferred to the printing medium 104 with a delay, the temperature of the heating element 113 itself is higher than the heat required directly for printing. Also, the thermal head 108 is adapted such that its momentary heat value per unit area is further increased and the heat generated by the heating element 113 is controlled to a higher and higher level in order to attain a higher speed of printing.
It should be noted that the resistance of the heating element 113 used in the thermal head 108 changes at a high temperature as will be seen in FIG. 3. As shown, the heating element 113 starts changing in resistance at a temperature T1 and will be broken down when arriving at a temperature T2. For faster printing, the printing medium 104 has to be moved correspondingly faster. Therefore, it is necessary that the heating element 113 should designed to provide a higher temperature. When the temperature becomes higher than the point T1, however, the heating element 113 will change in resistance with a change in heat value thereof, which will cause a print-density nonuniformity.
A technique for overcoming the above-mentioned drawbacks is disclosed in the Unexamined Japanese Patent Publication No. 59359 of 1990. This technique is to solve the aforementioned problem with the use of a combination of a thermistor and zener diode. Also, it is proposed in the Unexampled Japanese Utility Model Publication No. 39440 of 1994 to search a correction data table for correction data on the basis of resistance data and print-density gradation data, correct the energization of each unit heating element on the basis of the correction data and provide a print having a high gradation in density without being influenced by any change in resistance of the heating element. Further, the Unexamined Japanese Patent Publication No. 8502 of 1994 proposes to detect the temperature of a head or print sheet and increase the head or sheet carrying speed in case the detected temperature is higher than a temperature for a predetermined print density.
Incidentally, some of the thermal printers are designed to make margin-less print of image data on the printing medium 104. Such a thermal printer has to be designed to drive the heating element 113 of the thermal head 108 on a track whose width W2 is larger than a width W1 of the printing medium 104 as shown in FIG. 4. Thus, when such margin-less print is to be made, opposite end portions of the thermal head 108 will not be put in contact with the ink ribbon 105 and printing medium 104 as indicated with references 121. The heat of the thermal head 108 are also dissipated via the ceramic substrate 111, ink ribbon 105 and printing medium 104 with which the thermal head 108 is in contact. However, since the non-contact portions 121 are heat-insulated by air layer, it will not be able to dissipate the heat via the ink ribbon 105 and printing medium 104. Therefore, the temperature at the non-contact portions 121 will exceed the temperature T1 and further the temperature T2 as the case may be as shown in FIG. 3. When a dark portion such as a night scene or the like exists around an image, such a temperature is easily elevated because the heating element 113 has to provide a higher temperature. For a higher-speed printing, the heating element 113 has to provide a higher temperature so that the above temperature elevation is more likely to take place.
The sizes of the printing media 104 include various ones including L (89 mm by 127 mm) and KG (106 mm by 156 mm). Many of the ordinary printers are designed to make print on printing media 104 of more than one size. Here will be discussed serial printing including margin-less print on a small-size printing medium 104a as shown in FIG. 5A and print on a large-size printing medium 104b as shown in FIG. 5B. In this case, the non-contact portions 121 of the thermal head 108 used to make the margin-less print on the small-size printing medium 104a will be put in contact with the ink ribbon 105 and printing medium 104 as indicated with references 122. Being the non-contact portions 121 during the preceding print, the contact portions 122 are at a high temperature. So, when print is made on the large-size printing medium 104b, the ink in the ink ribbon 105 is sublimed excessively in the non-contact portions 121 alone to result in a high-density ink portion 123 in a printed image, which will cause a print-density nonuniformity. A change of only about 1% in resistance of the heating element 113 will make this print-density nonuniformity visible to the human eyes. Also, when the resistance is decreased, the power and heat value will increase, easily causing a print-density nonuniformity.
Further, the conventional thermal printers can do serial printing. However, such serial printing will cause the thermal head 108 to store the heat. After doing serial printing for a while after initial print, the thermal head 108 will get a higher temperature than that after the initial print. As a result, the density of a printed image will be too high.
To solve the above problem, there has been introduced a technique for decreasing the printing thermal energy which is to be applied to the conventional head 108 when the stored heat is larger. In this technique, consideration is given to the heat storage in the thermal head 108. In the case of a thermal printer, however, the stored heat causes the thermal head 108 to get a temperature approximate to the sublimation point of the ink, the sublimation ink in the ink ribbon 105 will sublime and transfer to the printing medium 104 even if no printing thermal energy is applied to the thermal head 108. It should specially be noted that in case an ink ribbon 105 and printing medium 104, both having a high sensitivity, are used for a higher-speed printing, the sublimation point will possibly be attainable with only the heat stored in the thermal head 108 before it is with the heat from the heating element 113.
The heating element 113 used in the thermal head 108 has such a physical property that the resistance thereof changes at a high temperature, as having previously been described with reference to FIG. 3. As a result, in case serial printing is done, the heating element 113 is continuously driven for a long time, so that the thermal head 108 will store heat. As a result, the heating element 113 will have the resistance thereof changed at a temperature higher than T1, so that the printing thermal energy of the heating element 113 will change, causing a print-density nonuniformity.
The Unexamined Japanese Patent Publication No. 58808 of 1999 discloses a technique for solving the above problem In the Publication, it is proposed to detect the temperature of the thermal head, interrupt energization of the thermal head when it is detected that the thermal head is overheated, and continuously feed the printing sheet with the energization being kept interrupted until the overheat is eliminated, to thereby dissipating the heat from the thermal head. Namely, the technique disclosed in the Publication is such that the overheat causing the print quality to be lower is eliminated by idly feeding the so-called printing medium to efficiently dissipate the heat stored in the thermal head via the printing medium and platen roller.
Therefore, the technique disclosed in the above Unexamined Japanese Patent Publication No. 58808 of 1999 makes it possible to efficiently cool the overheated thermal head and thus resume printing in a reduced wait time. In this case, however, the printing medium has to be reset before resuming the printing operation by reversing the idly forwarded printing medium to a print position where it was at the time of energization interruption. Therefore, even with this proposed technique, it is not possible to reduce the printing time sufficiently.
Especially, when many high-density images such as a night scene are printed at a high speed, the thermal head will have a large heat value, which will lead to frequent stop and cooling of the thermal head as well as to an increased length of time for which the user has to wait. Namely, the conventional thermal printer is not friendly to the user.